This invention relates to compressed air systems for automotive vehicles, and more particularly to a system employing and method of controlling a pair of air dryers for removing moisture from compressed air.
Transit coaches and air-assisted bulk-off load units are illustrative of compressed air systems used to operate the brakes of the vehicle, as well as to operate accessories associated with the vehicles. A standard air dryer cannot meet the demands of these systems. A desiccant material in the air dryer becomes overwhelmed and saturated, ultimately resulting in loss of function.
Continuous flow air dryers are required for air brake vehicle applications where high air use requires extended charging times and high compressor duty cycles. Continuous flow systems, therefore, typically use a pair of air dryers disposed in parallel between a source of compressed air and a storage reservoir. By occasionally purging air to exhaust from an air dryer, the dryer (specifically, the dessicant material) is regenerated and is more effective in removing moisture during its next duty cycle. Accordingly, prior systems employ a switching arrangement that alternates the pair of air dryers between charging the reservoir and purging, i.e., one air dryer is regenerated while the other air dryer supplies dry air to the reservoir.
Known competitive twin dryer arrangements function so that if the cycle is interrupted, the system starts anew. However, either a default arrangement establishes that the same air dryer begins the cycle or the cycle is random and there is a fifty percent (50%) chance that the saturated air dryer will be used to initially charge the reservoir upon startup. Thus, the same air dryer is used again and again for either a standard brake system charge cycle, i.e., normal brake system air use or with high air use associated with operating special vocational functions, such as air-assisted bulk off-loading, tire inflation, etc.
The normal brake mode involves charging the system reservoir used specifically for braking. Thus, there is a fifty percent (50%) probability that the previously used (and now saturated) air dryer cartridge will be the first one used for the next charge cycle. This increases the probability that wet air is introduced into the system--an undesirable result. Moreover, since one cartridge is used more frequently than the other, periodic maintenance associated with the twin dryers encounters one cartridge requiring service, and the other cartridge still available for use. Diagnostics for separately testing the individual cartridges is not presently available without additional test equipment. Therefore, to avoid the unnecessary expense of additional test equipment, and to maximize the productivity of a maintenance schedule, it would be preferred that both air dryers be serviced at the same time.